The water intended for distribution for urban or industrial uses is generally treated by means appropriate to minimize pollution, then stored and distributed.
In order to avoid organic developments, whether of algae, bacteria or pathogenic germs, it is necessary to inject a disinfectant, generally chlorine, upstream of a treatment or storage line, with a dosage such that the water is disinfected and that there remains at the distribution network inlet only a certain quantity of chlorine, called residual chlorine--generally from 0.05 to 0.2 mg/l.
A contact period is necessary for the disinfection of the water which has to be stored during that time, but the storage period can be variable, particularly so as to meet the downstream demand variations, while the upstream production is continuous.
The usual practice is to control more particularly the concentration of residual chlorine at the distribution network inlet, that is at the storage outlet, and to set in consequence the dosage of chlorine injected at the storage inlet.
This method is not satisfactory since one has to wait for a period at least equal to the storage duration in order to check that the desired result has been obtained for a given setting, the consumption of chlorine for the disinfection during storage not being predictable in advance; in particular, where there is a water quality deterioration upstream, causing a chlorine demand which is above what had been foreseen, one can find it out only long afterwards, when all the storage has been affected by the chlorine deficiency. When frequent water quality variations occur, an accurate setting is not possible and the dosage of chlorine has to be adjusted arbitrarily either to a level corresponding to the maximum demand, or to a medium level. In the first case, there is a risk of an excessive consumption of chlorine and a deterioration of the organoleptic qualities of drinkable waters during the periods where the demand is small; and in the second case the risk is that during the peak periods where the chlorine demand is high, the disinfection is insufficient and the network is polluted.